This invention relates to the heat treatment of cutting tools, in particular, although not necessarily exclusively, the heat treatment of cutting tools such as twist drills having a shank and a cutting portion to which it is desired to impart different hardness.
Cutting tools such as twist drills, milling tools, reamers, countersinks and the like include a cutting portion, formed with a number of cutting edges, and a shank by which the tool is held, for example in a collet chuck or other holder of for example a lathe, machine drill or hand drill. It is common practice to harden the cutting portion of these tools in order that they can cut efficiently. However, it is undesirable to harden the shanks to the same degree, because a relatively soft shank is required if the chuck or other holder is to grip the tool securely.
These cutting tools are typically manufactured from steel, most usually a high-speed steel. The process by which they are hardened is a heat treatment process, in which blanks for the tools are heated up to a temperature of about 1150-1230xc2x0 C., at which temperature they are held for a sufficient length of time to ensure that the blank is heated to its core. The blank is then rapidly cooled (i.e. quenched) to effect the change in microstructure that gives the steel its hardness. Hardening of other ferrous and non-ferrous metals can be achieved in a similar manner with suitable heat treatment regimes.
To give the desired differential hardening (fully hardened cutting portion/soft shank), the conventional approach is to use a salt bath for the heat treatment. The cutting portion of the tool is immersed in the liquid salt, which is held at the necessary high temperature. The shank remains clear of the bath and consequently remains at a temperature which is not sufficiently high for any appreciable hardening to occur.
The use of a salt bath in this way can reliably produce tools having the desired hardness characteristic, and is still the most common method of hardening used today. However, the process does have drawbacks, most notably the environmental and safety concerns associated with the toxic, extremely high-temperature molten salts used in the bath, which also give rise to difficult and unpleasant working conditions for the operator of the process.
More recently, it has been proposed to differentially harden cutting tools by treating them in a three-stage vacuum furnace, the tools progressing in a linear fashion through three chambers in the furnace. The tools are loaded in batches into the first chamber which is closed and then evacuated. After a predetermined amount of time, the batch of tools is then moved into the second chamber, which is already under vacuum, and which is held at a high temperature in order to heat the tools to the desired hardening temperature. Having been held in the heated chamber for an appropriate amount of time, the tools are then transferred to the third chamber. Here they are quenched by pumping nitrogen gas into the chamber under high pressure.
To achieve the desired differential cooling, the tools are held within the chambers of the furnace in carriers, in the form of large metal blocks formed with recesses in which the tool shanks are received. The carriers shield the shanks to some degree from the heated interior of the chamber. However, the temperature of the carriers themselves will increase, particularly where the heat treatment regime dictates that the tools must be held in the heating chamber for any significant length of time, possibly resulting in some unwanted hardening of the shanks. This problem can be exacerbated if the carriers are not allowed to cool sufficiently between batches of tools. The rapid cooling by blasting the tools with nitrogen may also lead to undesirable distortion.
Moreover, the furnace must be sealed from its surrounding environment, and within the furnace the three chambers must be separately sealed, in order that the necessary vacuum can be maintained, leading to a relatively complex and expensive design of furnace. It is perhaps for this reason that the salt bath still predominates, despite its drawbacks mentioned above.
The present invention has as its general aim the provision of heat treatment apparatus and methods for differentially hardening two portions of a cutting tool which offers an economic and reliable alternative to the conventional, and ever less desirable, salt baths.
In one aspect, the invention provides apparatus for heat treating a cutting tool, comprising a furnace within which there is at least one radiant heating element and a tool holder adapted to receive and shield a first portion of the tool from the heating element whilst a second portion of the tool is directly exposed to radiant heat from said element.
In another aspect, the invention provides a heat treatment method for hardening a metal tool, the method comprising directly exposing a first portion of the tool to a source of radiant heat in a furnace to raise the temperature of said first portion to an elevated temperature, and shielding a second portion of the tool from said source of radiant heat to maintain it at a temperature lower than the elevated temperature of said first portion.
The term xe2x80x9ctoolxe2x80x9d used herein is intended to include blanks and semi-finished blanks for tools as well as finished tools themselves.
By exposing the tools directly to a source of radiant heat it has been found possible to accurately control the differential heating of the two portions of the tool.
This control is enhanced when, as is preferred, the radiant heat source is arranged to lie alongside the tools when they are being heated in the furnace. In this case, it may also be arranged that the heat source, i.e. the heating element, does not extend alongside or at most extends only partially alongside the tool holder in which a portion of the tool is shielded. This further exaggerates the differential heating of the two portions of the tool.
Another particularly preferred measure to increase the temperature differential between the two portions of the tools, is to actively cool the tool holder. For instance air, water or some other cooling fluid may be forced through or around the tool holder or some other heat conducting element that is thermally coupled to the tool holder, whereby heat can be drawn away from the holder.
It is of course more economical to treat batches of tools at one time, and for this reason the furnace may be arranged such that a plurality of tools can be simultaneously exposed to the heating element. For instance, a row or two-dimensional array of tools may be held in one or more tool holders adjacent the element. To ensure a more uniform heating of the tools, two heating elements may be arranged, one either side of the tools, for example to lie parallel with a row of tools. This principle can be extended to layouts including two or more rows or arrays of tools extending parallel to one another, these rows or arrays being held in tool holders within corridors defined between opposed heating elements, e.g. three rows of tools held in three parallel corridors defined by four heating elements.
Where the tools are treated in batches, it is particularly preferred that each tool is directly exposed to radiant heat from at least one heating element, without being shielded or partially shielded from that element by any of the other tools of the batch. Typically, with the configuration of heating elements described above, this will mean that the tool holders should be arranged to hold at most two parallel rows of tools. Even then, it is desirable to offset the rows from one another such that the tools are fully exposed to the heating element to one side of the batch and only partially shielded from the element to the other side of the batch.
The furnace preferably also includes means for rapidly cooling the tool or tools subsequent to exposure to the heating element(s). Particularly preferred for this purpose are one or more cooling elements adjacent which a row or array of tools can be disposed in a tool holder, much in the same way as they are held alongside the heating element. The cooling elements, which may for example be cooled themselves by a flow of water or other cooling fluid, absorb heat radiating from the tools to help prevent the atmosphere around the tools increasing significantly in temperature, encouraging rapid cooling of the tools.
Similar to the heating elements, parallel rows of cooling elements may be arranged within the furnace to define one or more corridors for the tools.
Conveniently, the furnace may be divided into a heating zone in which the tools are heated by radiant heat and a separate cooling zone in which the tools are cooled, transport means being provided to take the tools from one zone to the other. A particularly convenient form of furnace that can be adopted for this approach is a rotary hearth furnace, in which the tools are carried by a rotating support or hearth, e.g. in their tool holder, through an annular chamber, which may be sub-divided into different temperature zones.